Abstract

Background

The Mollusca constitute one of the most morphologically and ecologically diverse metazoan
phyla, occupying a wide range of marine, terrestrial and freshwater habitats. The
evolutionary success of the molluscs can in part be attributed to the evolvability
of the external shell. Typically, the shell first forms during embryonic and larval
development, changing dramatically in shape, colour and mineralogical composition
as development and maturation proceeds. Major developmental transitions in shell morphology
often correlate with ecological transitions (e.g. from a planktonic to benthic existence
at metamorphosis). While the genes involved in molluscan biomineralisation are beginning
to be identified, there is little understanding of how these are developmentally regulated,
or if the same genes are operational at different stages of the mollusc's life.

Results

Here we relate the developmental expression of nine genes in the tissue responsible
for shell production – the mantle – to ecological transitions that occur during the
lifetime of the tropical abalone Haliotis asinina (Vetigastropoda). Four of these genes encode evolutionarily ancient proteins, while
four others encode secreted proteins with little or no identity to known proteins.
Another gene has been previously described from the mantle of another haliotid vetigastropod.
All nine genes display dynamic spatial and temporal expression profiles within the
larval shell field and juvenile mantle.

Conclusion

These expression data reflect the regulatory complexity that underlies molluscan shell
construction from larval stages to adulthood, and serves to highlight the different
ecological demands placed on each stage. The use of both ancient and novel genes in
all stages of shell construction also suggest that a core set of shell-making genes
was provided by a shared metazoan ancestor, which has been elaborated upon to produce
the range of molluscan shell types we see today.